1. Field of Art
The present invention relates to a heavy metal collection system and a heavy metal collection method for use of a substance produced in a wastewater processing to collect heavy metals from wastewater containing heavy metals.
2. Description of Relevant Art
Wastewater, such as industrial effluent, may contain heavy metals harmful to humans, e.g. mercury, lead, cadmium, chrome, etc. Such harmful heavy metals should be removed from wastewater.
As an example for removal of heavy metals from wastewater containing heavy metals (heavy-metal containing wastewater), there is a method of removing heavy metals from heavy-metal containing wastewater simply to meet a prescribed (effluent standard) value, and disposing of removed heavy metals as waste. Heavy metals of heavy-metal containing wastewater may be collected as valuable resources for recycle, to make effective use of heavy metals contained in wastewater.
Also as measures for collection of target substances such as heavy metals of wastewater, there are “unit processing techniques” for simple collection of target substances, and “composite techniques” as combinations of unit processing techniques for collection of target substances, among others, to be used as necessary for objectives.
As unit processing techniques there are e.g. “electrolysis method” and “alkaline precipitation method (coagulation sedimentation method)”. The electrolysis method has a pair of electrodes (anode, cathode) inserted in heavy-metal containing wastewater, for current conduction between the electrodes to deposit heavy metals thereon. The alkaline precipitation method has an alkaline chemical, such as caustic soda or slack lime, added to heavy-metal containing wastewater for sedimentation of heavy metals as hydrates.
As an exemplary method for composite techniques, there is a “sulfide method”. In the sulfide method, sodium sulfide or hydrogen sulfide is added to heavy-metal containing wastewater for precipitation and removal of heavy metals as sulfide salts. For use in the sulfide method, hydrogen sulfide may be generated from combination of sulfuric acid ions and organics by actions of sulfate-reducing bacteria under an anaerobic condition, for example. Sulfuric acid ions may be generated from simple sulfur by actions of sulfur-oxidizing bacteria, for example.
As a method of processing heavy-metal containing wastewater, there has been a technique disclosed in JP 2002-210436 A (Japanese Patent Application Laying-Open Publication No. 2002-210436) in which hydrogen sulfide is generated from a first wastewater containing sulfuric acid ions and organics by actions of sulfate-reducing bacteria, and added to a second wastewater containing heavy metals, for insolubilization of heavy metals as sulfide salts. The technique disclosed in JP 2002-210436 A is an effective method for separate supply of “a first wastewater containing sulfuric acid ions and organics” and “a second wastewater simply containing heavy metals”, where hydrogen sulfide from a processing of the first wastewater is available for processing the second wastewater.
There has been a technique disclosed in JP S62-193697 A (Japanese Patent Application Laying-Open Publication No. S62-193697), which generated hydrogen sulfide by sulfate-reducing bacteria and produced sulfide salts of heavy metals in identical system (an integrated reduction tank). In the technique disclosed in JP S62-193697 A, heavy-metal containing wastewater, sulfate-reducing bacteria, and nutrient salts for their growth are mixed together. And then, hydrogen sulfide are generated by actions of sulfate-reducing bacteria, at the same time heavy metals as sulfide salts are precipitated. In this system sulfide salts are used as adsorption carriers for proliferation of bacteria.
Generally, wastewater has a variety of contents. As a result, there are few cases where the technique disclosed in JP 2002-210436 A is applicable, that is, where “a first wastewater containing sulfuric acid ions and organics” and “a second wastewater simply containing heavy metals” must be separately and concurrently obtainable. JP 2002-210436 A describes techniques for removal of heavy metals from wastewater, without making consideration of heavy metals in insolubilized sulfide salts that may be valuable resources for collection of pure metals. JP 62-193697A employs sulfide salts of heavy metals as carriers, in no consideration of collecting heavy metals from sulfide salts, as valuable resources containing no impurities.
It is an object of the present invention to provide a heavy metal collection system and a heavy metal collection method for effective use of substances produced in a wastewater processing to collect heavy metals from wastewater containing heavy metals.